All optical systems suffer from image aberrations. The ultimate quality and performance of an optical system is determined by the extent to which aberrations, particularly chromatic aberrations, are corrected. In the present invention, significant improvement is achieved over the prior art by minimizing chromatic aberrations. Additional advantages are obtained by reducing the number of optical elements needed to achieve equivalent or superior results.
The present invention relates generally to improved structure for assemblies of optical systems in which refractive and diffractive optical elements are combined advantageously to achieve significant improvement in chromatic aberration. The diverse characteristic properties of these two types of optical elements are cancelled out by suitably opposing and balancing these properties against each other to achieve hitherto unobtainable image quality.
In the prior art, achromatic lens assemblies have been employed with limited success since the time of Isaac Newton. In these achromats, two optical elements (one with positive power and one with negative power) made from conventional glass materials (e.g. crown and flint glass) with differing dispersive characteristics are combined to reduce net chromatic aberrational effects. Ever increasing sophistication with both methods and materials has been applied to the fundamental problem of correcting chromatic aberration, and powerful computational methods combined with development of special glass materials have achieved significant improvement. It is typical of such effort, however, that ever smaller incremental gains are achieved at continually increasing cost.
In examples of the immediate prior art, the use of binary optics has been proposed (reference 1) to correct axial chromatic aberration in an infrared optical system and (reference 2) to correct primary lateral chromatic aberration without aggravating the secondary chromatic aberration of an Erfle eyepiece.
Reference 1 is a paper by Gary. J. Swanson and Wilfred. B. Veldcamp, SPIE Poceedings, Vol. 885, Paper #22, 1988. They propose using binary optics to correct axial chromatic aberration. In binary optics, the functional effect of a grating is achieved with grooves that are etched with typical fabrication processes employed in microelectronics. Their system has serious deficiencies, because as numerical aperture increases, marked spherochromatism is exhibited. Consequently, the minimum useable f-number is severly limited. This limitation is particularly unfortunate in advanced infrared optical systems that require low f-numbers (typically f/1.0 to f/1.5) to help reduce size and weight.
Reference 2 is by D. Shafer and T. McHugh of the Perkin-Elmer Corporation: "Binary Optics Design Surprises for the 1990's," SPIE, Orlando, Fla (March 1989). Binary optics are used to correct lateral chromatic aberration of an Erfle eyepiece without aggravating secondary chromatic aberration. Unfortunately, their approach is applicable only to systems with low numerical aperture (high f-number) and modest fields of view, as is characteristic of the Erfle eyepiece. As the aperture stop is opened, spherochromatism, chromatic coma, axial primary color, and axial secondary color are aggravated severely. Similarly, as field of view increases, chromatic distortion and chromatic coma are aggravated severely.